Hello,
I am just about to start building this amplifier. The circuit calls for using pairs of resistors in opposite polarities (If we could call it polarity being a resistor), from what I've read that may reduce/cancel inductance. I have never done that in any of my previous amplifier designs, and neither seems to be a common practice. Is that really necessary or did the designer done that the same way he could have used silver wire or isolation feet?
Does it really make a difference to make it worth it to implement that way?

All the resistors shown in parallel are available in film type which are completely non-inductive. Even wire-wound resistors have so little inductance that at audio frequencies the inductance is irrelevant.

. . . and there is NO guaranty that marking orientation has something to do with an improbable building assymetry . . .
I mean, how many operations beetween the metalisation and the marking processes and how many times the resistor "could have" been flipped ?

But yes, if they are inductive, putting two of them in parallel will halve the inductance this does not mean the 2 * 100K becomes less inductive than a single 50K was.
Anyway, the capacitance is always doubled

Andrea Ciuffoli is a member of the Forum, so he is the best person to answer your question.

Second thought.
You have 2 identical coils in parallel. (resistors). If you rotate one coil 180 degrees (as on the photo) you simply get the same circuit! To cancel magnetic field one coil should be wound clockwise and the other - anticlockwise.

I mean, how many operations beetween the metalisation and the marking processes and how many times the resistor "could have" been flipped ?

As already pointed out, flipping a helical coil end to end does nothing to change the coil's twist. So even if where is a magnetic property of such a resistor, it is not reversed by flipping the device end to end. You would literally have to unwind the coil and rewind with the opposite twist it to create the opposite magnetic effect - and that is not practical or possible unless you want to start winding your own resistors.